2-substituted naphth(1,2)oxazole scintillators



United States Patent Ofice 3,526,768 Patented Sept. 1, 1970 Int. Cl.G01t 1/20 US. Cl. 250-71 13 Claims ABSTRACT OF THE DISCLOSUREDESCRIPTION OF THE INVENTION This invention relates to a new class ofcompounds; namely, naphthyl substituted oxazoles and naphthoxazoles toinclude Q U N 2-(2naphthyl)-naphth(1,2)oxazole (NNO) 2- (phenyl) -naphth(1,2 oxazole In accordance with this invention, it has further beendiscovered that the foregoing compounds act as primary and/or secondaryscintillator solutes and, accordingly, one aspect of this inventionrelates to a process of enhancing the sensitivity of a system toradiation by incorporating in the system a small amount of the foregoingcompounds.

The method of this invention includes the determination of the radiationemission of a radioactive sample by conducting such determinations inthe presence of a solute consisting of a compound or mixture ofcompounds as aforedescribed.

Accordingly, it becomes a primary object of this invention to provide asnew compounds 2 (l naphthyl)- naphth(1,2) oxazole, 2 (2naphthyl)-naphth(1,2) oxazole, 2 (phenyl) naphth( 1,2) oxazole and 2-(2-furyl) -naphth( 1,2 -oxazole.

Another object of this invention is to provide a method of enhancing thesensitivity of a system to radiation by incorporating therein a smallamount, i.e., about 0.01 to 10 g. per liter, of 2 (1 naphthyl)naphth(1,2)oxazole, 2 (2 naphthyl) naphth(1,2)oxazole,2-(phenyl)-naphth(l,2) oxazole, 2 (2 furyl)-naphth( 1,2) oxazole andmixtures thereof.

Another object of this invention is to provide a process wherein thepulse heights and scintillation effects of solutions are enhanced formeasurements of these qualities by the incorporation of a small amount,preferably within the range of about 0.01 to 10 g. per liter, of 2- (lnaphthyl) naphth(l,2)oxazole, 2 (2-naphthyl)- naphth(l,2)0xazole, 2(phenyl) naphth(1,2)oxazole, 2 (2 furyl) naphth( 1,2)oxazole, andmixtures thereof, either as a primary or secondary solute.

Still another object is to provide solvent compositions for beta rayemission activity measurements comprising the compounds disclosed hereinand mixture-s of same.

These and other objects of this invention will be described or becomeapparent as the specification proceeds.

In order to demonstrate this invention, the following examples aregiven. In these examples a photomultiplier tube (PMT) of the end-windowtype manufactured by the Radio Corporation of America and bearingidentifying No. RCA6655 was connected through a preamplifiermanufactured by the Tracerlab Corporation bearing Tracer lab Type p-20D.The instrument Was set at full gain and connected to a pulse or countscaler having an input sensitivity of 0.25 volt. The PMT photo-cathodewas cooled to +10 C. to l5 C. to reduce noise pulses.

The solutes to be tested Were dissolved in toluene to concentrations ofabout 4 gm./liter and 0.5 ml. portions of these solution-s were placedin 1 ml. beakers containing 0.1 ml. of a standard solution of aradioactive isotope of iodine (I-13l) in toluene. The approximatespecific activity of the radioactive solutions thus produced was 0.01microcurie/ml. Solutions of toluene (0.6 ml.) and a mixture of 0.5 ml.of toluene and 0.1 ml. of the standard radioactive iodine solution werealso placed in one ml. beakers for comparison. The solutions were thenplaced, in turn, on the light sensitive window of the PMT, and thepulses counted. The results are shown in the following table.

The best contact between the radioactive sample and the scintillator isobtained when both the sample and scintillator are dissolved in the samesolvent, called the primary solvent. However, only a few substances havebeen found which act as scintilla'tors, and only a few solvents areknown having the ability to dissolve or suspend the sample and act totransfer the energy absorbed TABLE I Scintillation count, Run No. Sourcesample Primary solute Secondary solute c.p.m.

1 standard, comm. p-Terphenyl, 4.0 g./l. None 35, 237 33, 440

primary. 2 Comm. primary and ..do POPOP, 0.5 g./l 51,431 48,980

secondary. 3 C0rnm.sec0 dary None POPOP, 1.0 g./l 21, 042 .1 4 do "doPOPOP,O.5 g./l 163 5 2-(2-naphthylaphth- NNO, 4.0 g.[l No e 22,837

(1,2)oxazole (NNO). 6 NNO plus comm. NNO, 4.0 g./l POPOP, 0.5 g./l30,154 30, 113

seco dary. 7 Comm. primary, p-Terphenyl, 4.0g./l NNO, 0.5 g-ll 50, 72348, 950

NNO as seco dary.

The foregoing results shows scintillation tests run on NNO used as aprimary solute compared to p-terphenyl and NNO as the secondary solutecompared to POPOP (a commercial secondary solute). A comparison of Run 2with Run 7 reveals that NNO is equal to POPOP when used as a secondarysolute with p-terphenyl. A comparison of Run 1 with Run 5 shows that NNOis equal to 65 percent by weight of p-terphenyl when tested as a primarysolute alone. Further comparison of Runs 2 and 6 shows that NNO as theprimary solute is equal to about 60 percent of p-terphenyl as theprimary solute when both have POPOP as the commercial secondary solute.

The foregoing experiments bring up another advantage of this invention.It was found that the solubility of POPOP in toluene is limited to about1.0 g. per liter but the solubility of NNO in toluene is well over 4 g.per liter and extends up to 6 to 8 g. per liter. This makes NNO superiorto POPOP as either a primary solute or a secondary solute because theincreased solubility in toluene allows scintillation measurements to bemade under conditions of low temperature or low solubility.

It is often necessary to make such measurements at low temperatures, forinstance in the upper atmosphere during experiments conducted in theexploration of space or in arctic regions where extremely coldconditions are encountered. The increased solubility of NNO in theprimary solvent toluene, or in other primary solvents such as xylene,anisole, dioxane, 1,2-dimethoxyethane and ethylene glycol monoethyletherextends to temperatures as low as 100 C. The advantages of solubilityalso extend to mixtures of primary solvents such as 6 parts of dioxane,1 part of anisole and 1 part of 1,2- dimethoxyethane, or 5 parts ofdioxane and 1 part of ethylene glycol monoethylether.

The method of this invention is carried out using the new compounds ofthis invention as solutes for enhancing the pulse heights andscintillation effects in liquid scintillation or liquid phosphorcounting techniques. These techniques are relatively new, having beendeveloped in the past few years for the determination of beta activityof compounds, e.g., carbon 14 and tritium counting.

One of the main problems has been overcoming the factors which preventthe radiation from reaching the detector because of self-absorption,geometry, scatter, and absorption by air and counter windows. Threebasic requirements for good counting results are (l) the radioactivesample must be in good contact with the scintillator, (2) thescintillator must emit a strong flash of light, and (3) the countingmixture must be reasonably transparent to the light flashes.

from the beta particles to the scintillator. Also a great manysubstances, known. as quenchers, if present in the scintillatingsolution, inhibit this transfer of energy. Some of the primary solventsused are toluene, xylene, anisole, dioxane, 1,2-dimethoxyethane andethylene glycol monoethylether. Mixtures of these primary solvents arealso used, e.g., 6 parts of dioxane, 1 part of anisole and 1 part of1,2-dimethoxyethane, or 5 parts of dioxane and 1 part of ethylene glycolmonoethylether. These solvents can be used in the process of the instantinvention.

The two essential ingredients of the liquid scintillator are (1) thesolvent or primary solvent which functions to absorb the beta radiationand transfer it to (2) the scintillator or solute as it is also called.The literature on the solvents and solutes is often confusing both as toterminology and the effectiveness of various solutes or scintillators.This invention is based on the discovery of a class of compounds whichact as primary solutes or as secondary solutes for liquid scintillationor liquid phosphor counting.

A large number of liquid scintillator solutions have been investigatedand disclosed in the art, including the combination of p-terphenyl intoluene and such solutes as oligophenylenes, fluorenes, phenanthrenes,furans, benzoquinolines, 2-pyrones, oxazoles, thiazoles, benzoxazoles,pyrazolines, phenanthrolines, 1,3,4-oxadiazoles, the tetrazines,organometallics, esters of anthranilic acid and various otherheterocyclic compounds. Review of these prior art disclosures leavesmuch to be desired in the selection of a scintillator because themethods of evaluation are not standardized, different experimenters usedifferent techniques and different instrumentation. Accordingly, thisart is highly empirical and the selection of a good primary or secondarysolute or scintillation agent cannot be made on the basis of chemicalstructure and physical or chemical properties alone.

Furthermore, the technique is often diflicult where weak beta emittersare being counted and the background emission level is high. In liquidscintillation tests the procedure is to place a radioactive sample in avial containing a solvent such as toluene. A scintillating agent isadded which has the property of emitting light in the visible or nearultraviolet spectrum upon excitation from the radioactive sample. Thevial is placed adjacent the entry port of an instrument designed totransform a light signal to an electrical signal, such as aphotomultiplier tube. Low temperatures are used to reduce backgroundemission along with lead shielding and low counting volume. Under theseconditions, at each emission of the radioactive particle of the samplethe scintillating agent emits a pulse(s) of visible light which ischanged to an electrical pulse by the photomultiplier tube. The signalfrom the photomultiplier tube (PMT) is passed to a suitable measuringinstrument such as a scaler to indicate and/ or record the time andmagnitude of the PMT signal.

A secondary solute, such as 1,4-di[2-(5-phenyloxazolyl)]-benzene, whichcompound is known as POPOP, may be added for the purpose of transformingthe emitted light from the scintillating agent, which may be 2,5-di(4-biphenylyl)-oxazole, known as BB O, to a wave length detectable by thePMT. In these determinations the role of the scintillation solute is toemit a pulse of photons for each radioactive emission which depositsenergy in the solution and the solvent must absorb energy and transferit to the solute. A scintillation solute must, in addition to beingavailable and economical, be an efficient light emitter, produce aphoton spectrum which in a conventional scintillation detector will beeventually transmitted and reflected in the optical system and convertedinto electrical energy by the PMT, and be compatible with solubilityrestrictions imposed by the composition of the counting solution and bythe temperature at which the counting is performed.

The best light emitters and scintillating agents form emission spectraof too short Wave lengths to produce a photon spectrum which iseventually transmitted and re flected and accordingly, it isconventional practice to employ a two-solute combination comprising aprimary solute to insure a large number of emitted photons and asecondary solute which becomes the actual emitter and agent for controlof the spectrum of the photons.

In accordance with this invention, pulse heights and scintillationeffects of solutions have been found to be enhanced for measurements bythe incorporation of a small amount, preferably within the range ofabout 0.01 to gm./liter, of 2-(1-naphthyl)-naphth(1,2)oxazole,2-(2-naphthyl)-naphth-( l,2)oxazole, 2 (phenyl)-napl1th (1,2)oxazole,2-(2-furyl)-naphth(l,2)-oxazole and mixtures thereof.

The scintillating agents of this invention are used in apparatusdesigned to measure electromagnetic or corpuscular radiation fromnaturally occurring or artificially produced radioactive isotopes orfrom machineproduced radioactive isotopes or from machine-producedradiation. The scintillation solutes of this invention comprise a classof compounds which is distinguished by the characteristic that itsmembers lose a significant fraction of the energy from their excitationmolecules by the emission of light, which is measurable by means such asphotomultiplier tubes and associated equipment. The compounds of thepresent invention are not only eifective pulse height scintillationenhancers but are easy to prepare and quite inexpensive, these factorsrepresenting an advantage over the known pulse height enhancers such asPOPOP and BBO.

The compounds of this invention are used in the known manner as eitherprimary or secondary solutes, that is they may be used with such knownscintillators or primary solutes as p-terphenyl in toluene,2,5-diphenyloxazole (PPO), 2 phenyl 5(4-biphenylyl)-1,3,4-oxadiazole(PBD), 1,4-di-[2-(5 phenyloxazolyl)] benzene (POPOP) 2-(4-methoxyphenyl)-5-(4-biphenylyl)-1,3,4- oxadiazole, 2 (4 methoxyphenyl) 5(4-biphenylyl)- oxazole and the like. The solvents may be toluene,xylene, anisole, dioxane, 1,2-dimethoxyethane, ethylene glycolmonoethylether and mixtures thereof.

The method and solvent compositions of this invention are applicable inthe determination of beta ray emission activity of any radioactivesample including, but not limited to tritium, C H and C Na, K 1 Rb, lnNd Lu Re and the like involving primarily beta ray emission, that is,the emission of negative electrons resulting from the transformation ofneutrons into protons wherein there is an increase in nuclear charge byone unit, but no efiect on mass number. Various samples of materialshaving the foregoing atoms which are characteristic beta ray emitters,include a wide variety of organic compounds, and such specific compoundsas: urea, methanol, ethanol, acetylene, toluene, p-cymene, hexane,octane, acetic acid, caproic acid, phenylalanine and benzoic acidcontaining one or more C atoms; stilbene with an H atom; cholesterol andrelated steroids with H and C atoms, water with H atoms; water in urine,plasma, and the like with H atoms. Also environments containinginorganic compounds, such as, barium carbonate and sodium acetate with Catoms and potassium chloride with K atoms or salt with Na atoms can betested using the solutes of this invention.

In these determinations the known methods of sample preparation areapplied. Some scintillation samples may be solvents themselves, in whichcase it is only necessary to add the desired amount of solute orscintillation enhancer of this invention with or without an auxiliarysolute such as POPOP, i.e., at a concentration of .050.3 g./l. for mostdeterminations. Where the sample is soluble in the primary solvent, suchas toluene, dioxane and the like, which should be of the best qualityobtainable, at least reagent grade, it is only necessary to dissolve thesample therein in the desired concentration to form the stock sample andplace the stock sample solution in the vial of the instrument. If thesample is water or watersoluble, the procedure is to prepare the watersolution of the sample and mix it with absolute alcohol in the vial orcounting bottle. To this is added the stock solution of the solute inthe primary solvent to form a homogeneous mixture, allowing the maximumtoluene or solvent concentration consistent with the total volumedesired and the amount of sample necessary for the proper testing. Anyprecipitates which form during this procedure are filtered off andalcohol-washed, and the filtrate is combined with the homogeneousmixture. Those samples which are insoluble in a primary solvent, wateror alcohol are ground in a tissue homogenizer or a semi-micro ballmilland washed into the vial with the stock solute solution. Agitation isnecessary before counting is conducted.

The known procedures for scintillation counting are applied and it isnot considered necessary to elucidate thereon. A wide variety ofapparatus is available commercially for these determinations. Thus thesample holders and mounts may be of the type manufactured byNuclear-Chicago Corporation, Model M2A, the scintillation detector maybe Model DS55 and selfquenching G-M tubes or counters Models D22, D12,D50, D51 and D52, PMT devices and sealers from this source may be used,or the instruments for this purpose manufactured by TechnicalMeasurement Corporation or Packard Instrument Company may be used. Theprocedures outlined or detailed in various technical bulletins publishedby these companies may be followed in carrying out the method of thisinvention using the new solutes described herein.

The advantages of the low-temperature solubility of the oxazolecompounds of this invention are evidenced when scintillationmeasurements are made at temperatures of about 0 C. to 100 C. or undernatural environmental earth atmospheric temperatures which may be as lowas C. Where such measurements are made in spacecraft even lowertemperatures are contemplated. Laboratory bench scintillation apparatusare designated for low temperature use and for these purposes the use ofthe primary and secondary solutes of this invention extend the utilitiesto temperatures in the range of -15 C. to l00 C. as a feature of thisinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The method of determining the beta particle emission from aradioactive sample which comprises contacting said sample with asolution containing a primary solute of the group consisting of2-(1-naphthyl)-naphth 7 (1,2oxazole, 2-(2-naphthyl)-naphth(1,2)oxazole,Z-(phenyl)-naphth( 1,2)-oxazole, 2-(2-furyl)-naphth( 1,2) oxazole andmixtures thereof, said primary solute being present in an amountsufiicient to enhance the scintillation count and measuring the photonemission rate of said solution.

2. The method in accordance with claim 1 in which said primary solute ispresent in a concentration of about 4.0 g./liter.

3. The method in accordance with claim 2 in which said primary solute is2-(2-naphthyl)-naphth(1,2)- oxazole.

4. The method of determining the scintillation effect of a radioactivesample which comprises contacting said sample with a solution containinga primary solute and about 0.01 to about 10 gm. per liter of a secondarysolute of the group consisting of 2-(1-naphthyl)-napth (1,2)oxazole,2-(2 naphthyl)-naphth(1,2)oxazole, 2- (phenyl) napth(1,2)oxazole,2-(2-furyl) naphth(1,2)- oxazole and mixtures thereof and measuring thephoton emission rate of said solution.

5. The method in accordance with claim 4 in which said primary solutionis toluene and said secondary solute is present in a concentration ofabout 4 gm. per liter.

6. The method is accordance with claim 5 in which said secondary soluteis 2-(2-naphthyl)-naphth(1,2)oxazole.

7. The method of determining the beta particle emission from aradioactive sample at temperatures below about 15 C. to about 100 C.which comprises contacting said sample with a solution containing aprimary solute of the group consisting of 2-(1-naphthyl)-naphth- (1,2)oxazole, 2-(2-naphthyl)-naphth(1,2)oxazole, 2-(phenyl)-naphth-(1,2)-oxazole, 2-(2-furyl)-naphth( 1,2)- oxazole andmixtures thereof, said primary solute being present in an amountsufiicient to enhance the scintillation count and measuring the photonemission rate of said solution.

8. The method in accordance with claim 7 in which said primary solute isdissolved in a solvent of the group consisting of toluene, xylene,anisole, dioxane, 1,2-dimethoxyethane, ethylene glycol monoethyletherand mixtures thereof.

9. The method in accordance with claim 8 in which said solvent comprisesa mixture of 6 parts dioxane, 1 part anisole, and 1 part1,2-dimethoxyethane.

10. The method in accordance with claim 8 in which said solventcomprises a mixture of 5 parts dioxane and 1 part of ethylene glycolmonoethylether.

11. A scintillator solution for beta particle emission activitymeasurements comprising a primarysolute, a pulse height enhancing amountof a secondary solute of the group consisting of 2-(1-naphthyl)-naphth(1,2)oxazole, 2(2-naphthy1) naphth(1,2)oxazole, 2-(phenyl)naphth(1,2)oxazole, 2-(2-furyl)-naphth(1,2)oxazole and mixtures thereof,and a suitable solvent.

12. A scintillator solution in accordance with claim 11 in which saidsecondary solute is 2-(2-naphthyl)- naphth( 1,2 oxazole.

13. A scintillator solution in accordance with claim 11 in which saidsolvent is toluene.

References Cited UNITED STATES PATENTS 2,985,661 5/1961 Hein et a1.260-3092 FOREIGN PATENTS 567,665 12/1958 Canada.

OTHER REFERENCES Orr et al., J. Chem. Soc. (London), pp. 13374344,

Pushkina et al., Urals Polytechnic Institute, vol. 34,

No. 2, pp. 427-431, February 1964. ARCHIE R. BORCHELT, Primary ExaminerS. ELBAUM, Assistant Examiner US. Cl. X.R. 260-307

